1. Field of the Invention
The present invention relates to an optical pickup apparatus having a plurality of light sources of different wavelengths in order to record or reproduce data stored on a plurality of optical recording media. The present invention also relates to a method for manufacturing an optical pickup apparatus.
2. Description of Related Art
Different optical recording media, such as, a DVD, a CD and the like, have different track pitches. In order to record or reproduce data on these optical recording media, laser beams having different wavelengths are required. For example, in other to reproduce data on a DVD, a laser beam having a wavelength of 650 nm is required. Also, a laser beam having a wavelength of 650 nm can be used to reproduce data on an ordinary CD or a CD-ROM. However, for reproducing data on a CD-R, a laser beam having a wavelength of 650 nm cannot be used to properly reproduce the data because the CD-R has a low reflectance. In this case, a laser beam having a wavelength of 780 nm is used instead. For these reasons, an optical pickup apparatus for recording and reproducing data on optical media of different types is provided with two different laser beam sources that generate laser beams having wavelengths of 650 nm and 780 nm.
Japanese laid-open patent application HEI 9-120568 describes a two-light source type optical pickup apparatus. The optical pickup apparatus has a common semiconductor substrate and two laser diode chips formed on the semiconductor substrate. The laser diode chips generate laser beams having two different wavelengths. A prism having two reflective surfaces is mounted on the same semiconductor substrate, and laser beams emitted from the diode chips are guided into a common optical path, using the two reflective surfaces of the prism.
Also, Japanese laid-open patent application HEI 9-128794 describes a two-light source type optical pickup apparatus in which two laser beam sources and one common light-receiving device are provided. Lights reflected from a recording medium are guided into the common light-receiving device by using a common diffraction grating. Since the diffraction grating has wavelength dependency, positions of the two laser beam sources with respect to the optical axis are adjusted to offset the wavelength dependency, such that the lights can be received by the common light-receiving device.
Furthermore, Japanese laid-open patent application HEI 9-138967 describes an optical pickup apparatus having two light source units that emit laser beams of different wavelengths. Each of the light source units has a laser beam source and a light-receiving element that receives light emitted from the laser beam source and reflected from a recording medium.
The conventional two-light source type optical pickup apparatus described above have the following problems.
The optical pickup apparatus described in Japanese laid-open patent application HEI 9-120568 uses a prism having two reflective surfaces. Therefore, when one of the laser diode chips is positioned, the position thereof needs to be adjusted while the other laser diode chip is emitting light. An assembly work that requires such a positional adjustment takes a long time and an assembly machine for performing such an assembly work is costly.
Also, it is noted that optical films are formed to provide the two reflective surfaces on the prism. Since the prism that is mounted on the semiconductor substrate is very small, the adjustment of the film characteristics is difficult during the formation of the optical films. Furthermore, the prism is an optical component, which would likely have burrs and chips. Accordingly, it is difficult to provide a small prism with an optically effective aperture diameter that is required for the prism. Therefore, the manufacturing cost for a small prism having two reflective surfaces becomes high. Moreover, since such a prism is difficult to handle during assembly, the assembly cost becomes high.
Furthermore, it is noted that a three-beam method is generally used as a tracking error detection method for detecting tracking errors in reproducing data on CDs, and a differential push-pull (DPP) method is often used in the case of re-writable recording medium. When such a detection method is used, a diffraction grating for generating three beams is required for each of the different laser beams. However, the optical system described in Japanese laid-open patent application HEI 9-120568 can not dispose an independent diffraction grating for generating three beams for each of the different laser beam sources, or independently position or rotationally adjust the multiple light sources. Accordingly, the optical system described in the reference is applicable to a limited range of systems, and therefore cannot be widely used.
Next, in the optical pickup apparatus described in Japanese laid-open. patent application HEI 9-128794, the two laser beams having different wavelengths are transmitted through one common diffraction grating. Therefore, the grating surface of the diffraction grating needs a special structure that causes diffractive actions only for the respective wavelengths. For example, planes of polarization for the light sources are crossed at right angle, and a polarization dependent diffraction grating is used. The diffraction grating of this type is expensive, and therefore results in a higher cost for the apparatus.
Also, in the optical system described above, one of the laser beam sources concurs with the optical axis of the objective lens. However, the other laser beam source is disposed out of the optical axis of the objective lens. As a result, with respect to the light source that is disposed out of the optical axis of the objective lens, changes in the optical characteristic that take place as the objective lens moves to follow the tracks of a recording medium become different depending on directions of the movement of the objective lens. When changes in the characteristic during the movements of the objective lens are not symmetrical with respect to the optical axis, the system needs to be provided with certain countermeasures. Accordingly, the optical system described in this reference cannot be widely used.
Furthermore, the optical system described in Japanese laid-open patent application HEI 9-128794 uses a common diffraction grating. As a result, the condition for generating three beams cannot be adjusted separately or independently for each of the laser beams of the different wavelengths, in a manner that a recording track is interposed between the two laser beams of the different wavelengths with the laser beams converged on the recording track at desired angles. Therefore, the optical system described in this reference is applicable to a limited range of systems, and therefore cannot be widely used.
On the other hand, the optical pickup apparatus described in Japanese laid-open patent application HEI 9-138967 uses two sets of light source units, in which each of the light source units is internally mounted with a laser beam source and a light-receiving element. This results in problems, such as, higher apparatus costs as a whole. Moreover, the positional adjustment needs to be conducted for each of the optical elements in each of the light source units. These results in problems, such as, a longer time required for the adjusting work.
In view of the problems of the conventional two-light source type or multiple-light source type optical pickup apparatuses, it is an object of the present invention to provide an optical pickup apparatus that is widely applicable to many different systems, and that can readily adjust positions of its optical elements without increasing the cost.
In accordance with one embodiment of the present invention, an optical pickup apparatus has a light source unit that includes a first light source and a common light-receiving device having first and second light-receiving sections.
The optical pickup apparatus may further include other optical devices, such as, an optical path composition element that guides first light emitted from the first light source and second light emitted from another light source into an optical path that passes an objective lens, and an optical path polarization element that guides reflected lights of the first and the second lights to the common light-receiving device.
In one aspect of the embodiments of the present invention, the optical path polarization element may be mounted in the light source unit for guiding reflected lights of the first and second lights to the common light-receiving device.
In another aspect of the embodiment of the present invention, the first and second light-receiving sections of the common light-receiving device may be formed on a semiconductor substrate. The first and second light-receiving sections are formed from first group and second group of light-receiving elements, respectively, that receive reflected lights of the first light and the second light, respectively.
In this manner, in accordance with the embodiment of the present invention, one of the light sources mounted in the light source unit that has the common light-receiving device. Accordingly, light emitted from the other light source and reflected on a recording medium is also received through the optical path polarization element by the common light-receiving device in the light source unit. Therefore, an optical pickup apparatus can be equipped with a single light-receiving device for receiving multiple light beams. As a result, the cost for the apparatus is lowered, compared with the conventional apparatus that uses multiple light source units including multiple light-receiving devices.
Furthermore, a diffraction element for generating three beams can be disposed in each of the spaces between the light source unit and the optical path composition element and between the second light source and the optical path composition element. As a result, the diffraction elements can be adjusted independently from one another, such that three beams provided by each of the diffraction elements can be irradiated on a recording track at any angles. Therefore, the apparatus of the embodiment of the present invention is applicable to a wide range of different systems.
Moreover, with the apparatus of the embodiment of the present invention, an optical axis of a laser beam emitted from each of the light sources can be adjusted to concur with an optical axis of the objective lens. As a consequence, changes in the optical characteristic of the objective lens accompanied with movements of the objective lens are symmetrical about the optical axis. Therefore, the apparatus of the embodiment of the present invention does not require a special signal compensation circuit that is required when such changes are asymmetrical.
The optical path polarization element may be a diffraction type optical element. The diffraction type optical element may be mounted on the light source unit to form an integral part of the light source unit. The diffraction type optical element may be rotatably adjustable about an optical axis of the first light emitted from the first light source. As a result, the work for adjusting the angular positions of these elements is readily performed.
Furthermore, in accordance with the embodiment of the present invention, the optical pickup apparatus may be provided with a positioning member that supports the light source unit, in a manner that an optical distance between the light source unit and the objective lens is adjustable. As a result, differences in the optical distance can be compensated in a single light source type optical pickup apparatus that may be provide by removing the second light source and the optical path composition element from the apparatus described above.